Add jsbundle property to global metadata

Adding `jsbundle` property to the global metadata with a structure `jsbundle: { 'source-file.js': { src: 'destination-file.js', async: }}}`.

This allows templates to do:

```njk
{% for script in jsbundle %}

{% endfor %}
```

As well as use a mechanism whereby file front-matter specifies the scripts to load:
```yaml
---
scripts:
- 'assets/js/common/source.js'
- 'assets/js/app/source.js':
async: true
integrity: 'abcdefg'
---
```
```njk
{% for script in jsbundle %}
{% if typeof script === 'string' %}

{% else %}

{% endfor %}
```

Start by tracing how global metadata and file front matter are currently assembled and passed to templates. Define the jsbundle shape for string and attribute-map entries, then verify that front-matter script declarations produce the expected script attributes in rendered templates.

Written by the indexing model from the issue text.